An organic semiconductor material having a charge transporting function is useful as a semiconductor film forming material for an organic device such as an organic electrophotographic photoconductor, organic EL device, organic TFT or organic solar battery.
As a method for allowing a resin to have such a charge transporting function, there is a typical method of dispersing a charge transport material in a resin used as a binder for formation of a functional film, etc. For example, this method is widely used for electrophotographic photoconductors.
In recent years, due to the reduction in the size of photoconductors necessitated by the reduction in the size of electrophotographic apparatuses or the increase in the speed of the electrophotographic apparatuses, securing quick responsivity and stability of the photoconductors has been a very important aim.
Examples of commercially available charge transport materials include 1,1-bis(p-diethylaminophenyl)-4,4-diphenyl-1,3-butadiene (refer to PTL 1), 5-[4-(N,N-di-p-tolylamino)benzylidene]-5H-dibenzo[a,d]cycloheptene (refer to PTL 2), 9-methylcarbazole-3-aldehyde 1,1-diphenylhydrazone, pyrene-1-aldehyde 1,1-diphenylhydrazone (refer to PTL 3), 4′-bis(4-methylphenyl)amino-α-phenylstilbene, N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine and 9,9-dimethyl-2-(di-p-tolylamino)fluorene.
A general charge transport layer is a solid solution film of approximately 10 μm to approximately 30 μm in thickness, obtained by dispersing any such low-molecular charge transport material in a binder resin.
As the binder resin, a bisphenol polycarbonate resin, a polyarylate resin or a copolymer composed of such a resin and other resin is used in most electrophotographic photoconductors. However, these charge transport materials do not have responsivity which can sufficiently adapt to a higher processing speed expected soon.
NPL 1 shows guidelines on molecular design in relation to quick responsivity (high mobility) of such charge transport materials. Specifically, NPL 1 states that a phenylamine group (>N-phenyl) serves as a functional group, there is a clear correlation between the number of functional groups and the mobility, and the mobility increases as the number of functional groups in a molecule increases; the compound of the present invention, which realizes quick responsivity by including a large number of functional groups, conforms to what is reported in NPL 1 on this point.
Meanwhile, the following have been reported: use of a tetrahydropyranyl group yields superior solubility in solvent and superior compatibility with a binder resin such as a polycarbonate resin; and low crystallinity of a charge transport material itself yields superior stability of a coating film and a coating liquid (refer to PTL 4). However, in the case where a tetrahydropyranyl group is included and an asymmetric molecular structure is employed, a low-viscosity liquid substance is easily produced. The low-viscosity liquid substance is superior in coating stability but poorly compatible with binder resins such as polycarbonate resins and easily degrades in terms of fingerprint resistance when stored for a long period of time. Moreover, the low-viscosity liquid substance easily degrades in terms of chargeability.